1. Field of the Invention
The present invention relates to an optical-receiving apparatus including an avalanche photodiode and particularly relates to an optical-receiving apparatus and a bias-voltage-control method-used for the optical-receiving apparatus that are provided to control a bias voltage applied to the avalanche photodiode according to the ambient temperature of the avalanche photodiode so that a multiplication factor used for the avalanche photodiode becomes a predetermined multiplication factor.
2. Description of the Related Art
In many cases, an optical-receiving apparatus including an avalanche photodiode (hereinafter often referred to as an APD), as a photoreceptor, is used for an optical-communication apparatus.
It has become increasingly preferable that optical-transmission-and-reception apparatuses including the above-described optical-receiving apparatus be in conformity with a standard multi-source agreement (MSA) stipulated, so as to supply devices with stability. The term “MSA” denotes an industry format determined to establish a system used to supply products with stability by providing commonality among the package sizes, pin configurations, interface specifications, and so forth of products between a plurality of vendors.
A monitor signal generated by monitoring the intensity of an optical signal transmitted to the optical-receiving apparatus is defined by the above-described MSA. While the optical-receiving apparatus is operated, a user makes a request to monitor the optical signal transmitted to the optical-receiving apparatus at all times by using the monitor signal. For responding to the user's request, a high degree of monitor accuracy should be achieved.
An optical-receiving apparatus disclosed in Japanese Unexamined Patent Application Publication No. 11-40840 includes an APD configured to generate the optical-signal current corresponding to the intensity of an optical signal transmitted to the APD. Subsequently, the optical-receiving apparatus monitors the optical-signal current flowing through the APD by using a current-mirror circuit.
It is preferable that the optical-signal current flowing through the APD operate linearly at all times with reference to the intensity of the optical signal-transmitted to the APD. In a range where the intensity of the optical signal is high, the optical-signal current is proportional to the optical-signal intensity. In a range where the optical-signal intensity is low, an ordinary optical-receiving apparatus such as an optical-receiving apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-217833 controls a bias voltage applied to the APD so that a multiplication factor is increased, so as to optimize the signal-to-noise ratio. Subsequently, in the range where the optical-signal intensity is low, the optical-signal current becomes nonlinear with reference to the optical-signal intensity, which makes it difficult to monitor the intensity of the transmitted optical signal with accuracy.
However, according to Japanese Unexamined Patent Application Publication No. 2004-289206, the relationship between the intensity of an optical signal transmitted to an APD and an optical-signal current flowing through the APD is made to be an almost linear relationship according to a change in the ambient temperature of the APD, which gives an answer to the problem described in Japanese Unexamined Patent Application Publication No. 2002-217833.
For optical-receiving apparatuses including an APD, there has been disclosed at least one technology which allows for establishing the relationship between the intensity of input light transmitted to the APD and an optical current (monitor current) flowing through the APD, as an almost linear relationship. In that case, however, the optical current (monitor current) becomes nonlinear as a multiplication factor changes due to temperature fluctuations.